This invention relates to a system for predicting bias effects on an actuator arm in a disc drive system using a lookup table.
There has been an effort to increase the radial density of concentric data tracks in high performance hard disc drives. As a result, there is an increasing need for higher order modeling and compensation for previously overlooked mechanical and electrical non-linear behavior in the actuation system used to position a head over a selected track of the disc. One such non-linear effect is associated with actuator bias. A rotary voice coil-driven actuator experiences variable amounts of torque (bias) at various radial positions relative to the disc. This bias is a result of several factors, including spring action operating on the actuator due to the flexible cable connecting the arm electronics to the disc drive electronics, windage acting on the arm, hysteresis friction in the actuator bearings, and other factors. Thus, the current required to maintain the head over the selected track of the disc might vary due to the bias.
During tracking, it has become common to compensate for direct current (DC) bias effects by applying a fixed bias current to the actuator arm. While the fixed bias provides reasonably good compensation for tracking operations after long seeks, it is not altogether accurate for tracking after short seeks that follow direction reversals, especially as radial track density increases requiring greater precision. The fixed bias model typically consists of a pair of second order polynomial curves, one each for seeks in the inbound and outbound directions. However, for relatively short seeks, the second order model may be insufficient to precisely predict and model the bias characteristics of the disc drive system, thereby increasing the time required to perform a track seek and settle on a desired track of the disc. The closer the predicted value of bias is to the actual amount of bias necessary to center the head over the selected track, the less settling time is required.
The shortcomings of second order polynomial models are specifically addressed in "A State-Space Bias Model for Prediction of Actuator Tracking Bias in Hard Disk Drives" by Kyle Eddy and William Messner (presented and distributed at the November 1995 Winter Annual Meeting of the ASME). The Eddy et al. paper reveals that the physical bias curve forms a hysteresis loop for short seeks following a change in actuator direction, which can itself be modeled as fitted exponential curves. However, overshoot and undershoot effects still existed in the actuator bias following a reversal in actuator direction, which could not be accurately predicted even by the exponential model.
Therefore, a need exists for a system which introduces bias into an actuator assembly of a disc drive that accounts for hysteresis, overshoot, undershoot, and other bias characteristics, improves tracking performance, and reduces the time required to settle over a desired track.